Retaining walls are a common method of retaining soil. The requirements of a functional retaining wall are structural stability and durability against the exposed environment and the provision of drainage.
To complete a retaining wall, typically earth is excavated, a block wall or concrete wall is erected, gravel is bagged and stacked adjacent the block wall, weep holes are located in the wall at various locations along the length of the wall, pipes are located through the weep holes to drain any water that passes through the gravel to the base of the wall, and the excavated soil is placed behind the retaining wall.
It is important that the drainage system functions correctly as retaining walls are rarely designed to withstand ponded water pressure. Ponded water pressure can triple or quadruple the loading on the retaining wall if allowed to develop. Often this additional pressure will cause the retaining wall to fail with serious consequences.
The stacked bags of gravel located behind the wall provide a drainage system. However, sometimes a bag may split. The pressure of the earth then spreads the earth through the gravel preventing water from flowing through the gravel reducing the efficiency of the drain. If many bags spit then ponded water may occur which is undesirable.
Another problem with using bagged gravel is that it is very time consuming to produce the drainage system. Gravel must be individually placed into bags by shovelling the gravel into a bagging machine. The bags must then be stacked on top of each other. The making and placing of the bags of gravel usually take considerably more time than the construction of the block wall.